This invention relates to battery systems which are adapted to operate under a stabilized temperature condition.
Complex and sensitive electronic equipment is finding an ever expanding variety of applications. The advent of solid state microelectronic technology enables this electronic equipment to be operated effectively at low voltage and under small or moderate current drain. That fact makes it possible to power the equipment with a rechargeable battery source under certain conditions. A familiar example is the case of digital computers in which a battery is used as a standby power source to protect the computer memory against erasure. During normal operation the computer is operated from line power. A standby power source, supplied by rechargeable batteries, is automatically switched into the power circuit in the event of line power failure. Under normal operation conditions, when the line source is operative, the standby battery is maintained in a charged condition by a low level, usually continuous charging current.
Computers and other electronic circuits may be subjected to temperature extremes which would impair the performance of the standby battery and/or reduce its life expectancy. The harmful influence of excessive heat or cold on, for example, nickel-cadmium cells (a frequently used type of rechargeable cell) is known. These cells incorporate electrolyte absorbent separators between the positive and negative electrodes. The separators are fabricated from fibrous or porous materials such as nylon or polypropylene. Nylon separators are known to oxidize at high temperatures, resulting in a loss of chemical and physical integrity. Additionally, charge acceptance is significantly reduced at high temperatures, while charge voltage and charge retention are depressed. Because of the depression in the terminal voltage relative to a fixed end-of-discharge cutoff voltage, the available cell capacity is also reduced. For any given cell design, capacity reduction becomes increasingly significant as discharge current increases.
Polypropylene separators are able to withstand considerably higher temperatures than nylon separators, and this reduces short circuit failure due to separator failure. Nevertheless, the performance limitations at elevated temperatures noted above are still present. Thus, the chemical effects at elevated temperatures degrade the cells' electrical performance irrespective of the type of separator used.
Rechargeable cells also suffer in performance at low temperatures. One of the most serious limitations is the rise in internal cell resistance at low temperatures due to diminished electrochemical activity and the reduced electrolyte mobility. This results in a lower output voltage and less discharge current. Charging performance, on the other hand, is also affected. At low temperatures the plate recombination capability of the system cannot balance the input charge rate, and this causes a rise in the cell's internal gas pressure. If the gas pressure builds up to a point exceeding the venting threshhold, the safety vent valve opens and a certain amount of electrolyte is lost with each vent opening.
Electrochemical cells can be designed to improve their ability to perform at and withstand temperature extremes. Thus, it is possible to design a cell so as to alleviate some of the above-noted limitations. If such a cell is designed for high temperature, however, its low temperature performance is generally even further degraded, and vice versa. It is difficult, if not impossible or impracticable, to design a cell so that it achieves adequate performance at both temperature extremes.